scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

 diff -durN gcc-4.4.0.orig/gcc/stmt.c gcc-4.4.0/gcc/stmt.c

 --- gcc-4.4.0.orig/gcc/stmt.c	2009-02-27 20:49:42.000000000 +0100

 +++ gcc-4.4.0/gcc/stmt.c	2009-05-27 21:38:21.000000000 +0200

 @@ -2376,7 +2376,11 @@

  	  use_cost_table

  	    = (TREE_CODE (orig_type) != ENUMERAL_TYPE

  	       && estimate_case_costs (case_list));

 -	  balance_case_nodes (&case_list, NULL);

 +	  /* When optimizing for size, we want a straight list to avoid

 +	     jumps as much as possible. This basically creates an if-else

 +	     chain.  */

 +	  if (!optimize_size)

 +	    balance_case_nodes (&case_list, NULL);

  	  emit_case_nodes (index, case_list, default_label, index_type);

  	  if (default_label)

  	    emit_jump (default_label);

 @@ -2942,6 +2946,7 @@

  	    {

  	      if (!node_has_low_bound (node, index_type))

  		{

 +		  if (!optimize_size) /* don't jl to the .default_label. */

  		  emit_cmp_and_jump_insns (index,

  					   convert_modes

  					   (mode, imode,